Site specific mutations in the metallothionein coding sequence will be followed for their expression. Selected codons will be specifically altered to give rise to amino acid replacement from non-aromatic to aromatic. The latter is normally absent in all native metallothioneins thus far analyzed. The possibility that the absence of aromatic amino acids is due to mutational probability of unfavorable processing at the transcriptional or translational level will be examined in vivo and in vitro. The altered metallothionein if obtained will be compared with the function and properties of those occurring naturally.